To deal with the need for lightening the weight of automobiles, which began back with the old oil crises, the steel sheet used for automobiles has been made higher in strength. In recent years, a need has arisen for improving the safety of automobiles in collisions. The need is rising for further increasing the strength of steel sheet for automobile use. As steel sheet rises in strength, generally its shapeability is sacrificed and cracks easily form at the time of press-forming. For this reason, in application of high strength steel sheet, in the press-forming step, it is necessary to work to prevent cracking. In particular, in press-forming steel sheet for automobile use, “punching hole expansion” for stretching the punched end face in the circumferential direction is often seen. It is important to prevent cracking due to such shaping.
FIGS. 1A and 1B show the state of deformation of a worked material in a punching process using a conventional flat-bottomed punch. In this punching process, a large compressive or tensile stress is applied to the hardened layer shown in FIG. 1B, so that part is remarkably hardened. Further, for this reason, the ductility of the end face after punching deteriorated and the punching hole expandability of the punched hole remarkably deteriorated. The deterioration of the ductility of the end face due to this hardened layer is particularly remarkable in high strength steel. Improvement of the punching hole expandability of high strength steel sheet now being made much use of to deal with the recent needs for lightening the weight of automobiles is being sought.
As an art for punching for improving this punching hole expandability, the art of punching using a punch with a projection attached to its tip has been proposed (PLTs 1 and 2).
The principle of improvement of the punching hole expandability by this art will be explained below. The punching hole expandability deteriorates in the punching process due to the formation of a work hardened layer at which the plastic strain accumulates. In the punching process, first, the punch and die shear the worked material whereby a shear plane is formed. Next, a crack occurs and advances in the worked material near the cutting edges of the punch and die resulting in a fracture surface. This crack grows and leads to fracture. The plastic strain on the worked hardened layer of the fracture surface mainly occurs at the stage of formation of the shear plane, so the shorter the stage of formation of the shear plane, the smaller the plastic strain on the work hardened layer of the fracture surface. Due to the effect of the projection attached to the punch, tensile stress is given to the punched worked material in the vicinity of the cutting edges of the punch and die, the advance of a crack is promoted, and as a result it is possible to shorten the stage of formation of the shear plane. Due to this effect, the strain at the punched end face is suppressed and the punching hole expandability is improved.
In PLT 1, furthermore, to prevent shearing of the material due to the projection, making the radius of curvature Rp of the shoulder of the projection 0.2 mm or more or making the projecting shoulder angle from 100° to 170° are made requirements. In PLT 2, to improve the ductility of the end face of the worked material, making the angle of the tangent drawn from the punch cutting edge to the projection 3° to 70° is made a requirement.